Photoflash lamp and method of coating same

ABSTRACT

A photoflash lamp having a vacuum-formed thermoplastic coating for reinforcing the glass envelope of the lamp and enhancing its containment capability upon flashing. Also, a method for applying the coating comprising: locating the glass envelope in a dried, preformed sleeve of thermoplastic material; drawing a vacuum in the space between the sleeve and the envelope; heating the assembly incrementally lengthwise to gradually form the sleeve onto the envelope; and then constricting and tipping off the sleeve at the conclusion of the heating process.

United States Patent 1 1 Rough et al.

[ 1 PHOTOFLASH LAMP AND METHOD OF COATING SAME [75] Inventors: Harold L.Hough, Beverly; Emery G. Audesse, Salem; Thomas J. Sentementes,Wakefield, all of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers. Mass.

[22] Filed: Sept. 4, 1973 [21] Appl. No.1 394,106

Related U.S. Application Data [62] Division of Ser. No. 268,576. July 3.1972.

[52] U.S. Cl. 156/286; 156/287; 156/294;

264/92; 264/102; 264/272; 431/93 [51] Int. Cl. B6513 31/00 [58] Field 01Search 156/86, 160, 165, 193,

156/212, 213, 214. 229. 215. 267, 285, 286, 287, 293,294, 303.1. 311,289, 104; 264/271, 272, 319, 327, 90, 92. 102; 53/22 B; 215/12 A, 12 R,11 E, 246; 431/94, 95;

[56] References Cited UNITED STATES PATENTS 3.313.084 4/1967 Forman53/22 13 [4 1 July 15, 1975 3,454,443 7/1969 Zafiroglu 156/287 3,463,0598/1969 156/287 3,470,046 9/1969 Verdin 156/86 3,514,081 5/1970 Cavanaughet a1. 156/86 3.551.258 12/1970 Galvanoni et a1 156/287 3,689,339 9/1972Klingler 156/3031 FOREIGN PATENTS OR APPLICATIONS 1.935.287 7/1969Germany 156/86 1,193,364 1/1966 Germany Primary Examiner-Charles E. VanHorn Assistant Examiner-F. Frisenda, Jr. Attorney, Agent, or Firm-Edward.1. Coleman [5 7] ABSTRACT A photoflash lamp having a vacuum-formedthermoplastic coating for reinforcing the glass envelope of the lamp andenhancing its containment capability upon flashing. Also. a method forapplying the coating comprising: locating the glass envelope in a dried.preformed sleeve of thermoplastic material; drawing a vacuum in thespace between the sleeve and the envelope; heating the assemblyincrementally lengthwise to gradually form the sleeve onto the envelope;and then constricting and tipping off the sleeve at the conclusion ofthe heating process.

25 Claims, 7 Drawing Figures PHOTOFLASI-I LAMP AND METHOD OF COATINGSAME This is a division of application Ser. No. 268,576 filed July 3,1972.

BACKGROUND OF THE INVENTION This invention relates to photoflash lampsand, more particularly, to a protective coating for flashlamps and amethod for applying such a coating.

A typical photoflash lamp comprises an hermetically sealed glassenvelope, a quantity of combustible material located in the envelope,such as shredded zirconium or hafnium foil. and a combustion supportinggas, such as oxygen, at a pressure well above one atmosphere. The lampalso includes an electrically or percussively activated primer forigniting the combustible to flash the lamp. During lamp flashing, theglass envelope is subject to severe thermal shock due to hot globules ofmetal oxide impinging on the walls of the lamp. As a result, cracks andcrazes occur in the glass and, at higher internal pressures, containmentbecomes impossible. In order to reinforce the glass envelope and improveits containment capability, it has been common practice to apply aprotective lacquer coating on the lamp envelope by means of a dipprocess. To build up the desired coating thickness, the glass envelopeis generally dipped a number of times into a lacquer solution containinga solvent and a selected resin, typically cellulose acetate. After eachdip, the lamp is dried to evaporate the solvent and leave the desiredcoating of cellulose acetate, or whatever other plastic resin isemployed.

In the continuing effort to improve light output, higher performanceflashlamps have been developed which contain higher combustible fillweights per unit of internal envelope volume, along with higher fill gaspressure. In addition, the combustible material may be one of the morevolatile types, such as hafnium. Such lamps, upon flashing, appear tosubject the glass envelopes to more intense thermal shock effects, andthus require stronger containment vessels. One approach to this problemhas been to employ a hard glass envelope, such as the borosilicate glassenvelope described in U.S. Pat. No. 3,506,385, along with a protectivedip coating. Although providing some degree of improvement in thecontainment capability of lamp envelopes, the use of dip coatings andhard glass present significant disadvantages in the areas ofmanufacturing cost and safety. More specifically, the hard glass incursconsiderable added expense over the more commonly used soft glass due toboth increased material cost and the need for special lead-in wires toprovide sealing compatibility with the hard glass envelope. In addition,even through more resistant to thermal shock, hard glass envelopes canalso exhibit cracks and crazes upon lamp flashing, and, thus, do notobviate the need for a protective coating.

In the typical dipping process for applying protective coatings, a largenumber of flashlamps are loaded on a rack and then successively dippedin a solvent solution and oven dried three or four times to build up thedesired coating thickness. Such a process is time consuming, uses arelatively large area of production floor space, and involvesconsiderable hand labor, all of which add significantly to manufacturingcost. Further, as the lacquer solution includes a highly flammablesolvent, such as acetone, an inadvertant flashing of one of the lamps ineither the dip bath or drying oven can ignite the solvent fumes. Tosubstantially reduce or eliminate this hazard, costly automaticextinguishing equipment must be employed. In the event of a solventignition, the resulting downtime and consumption of fire extinguishingchemical also adds to the manufacturing cost.

Application of the protective coating by means of a clipping process canalso preclude the use of more de sirable reinforcing materials. Forexample, a much stronger containment vessel could be provided by the useof a polycarbonate coating, due to its higher impact strength and highersoftening temperature, as compared to cellulose acetate. By using theconventional dipping and drying process to apply polycarbonate, however,a relatively cloudy coating results. In order to obtain a clear,transparent coating, an extremely low humidity must be maintained in thedrying ovens, which in turn requires the drying of 5,000 to 10,000 cubicfeet of air per minute. The incorporation of such a drying operationwould be prohibitively expensive.

SUMMARY OF THE INVENTION In view of the foregoing, a principal object ofthe invention is to provide a photoflash lamp having a stronger envelopestructure for providing improved containment during flashing.

Another object of the invention is to economically provide an improvedcontainment vessel for a flashlamp.

A further object of the invention is to provide an improved method forcoating the glass envelope of a photoflash lamp with a thermoplasticmaterial.

These and other objects, advantages and features are attained, inaccordance with the invention by vacuum forming a thermoplastic coatingon the exterior surface of the glass envelope. The improved methodcomprises: placing the glass envelope within a performed sleeve of thethermoplastic material; drawing a vacuum in the space between thethermoplastic sleeve and the glass envelope; and, simultaneously heatingthe assembly incrementally along its length, whereby the temperature andvacuum cause the thermoplastic to be incrementally formed onto the glassenvelope with the interface substantially free of voids, inclusions andthe like.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fullydescribed hereinafter in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an enlarged sectional elevation of an electrically ignitablephotoflash lamp having a protective coating in accordance with theinvention;

FIG. 2 is an enlarged sectional elevational of a percussive-typephotoflash lamp having a protective coating in accordance with theinvention;

FIG. 3 is an enlarged sectional elevational of a preformed sleeve ofthermoplastic adapted for assembly and vacuum forming onto the glassenvelope of a percussive-type photoflash lamp;

FIG. 4 is an enlarged elevation, partly in section, showing a percussiveflashlamp assembled in the thermoplastic sleeve of FIG. 3, prior tovacuum forming.

FIG. 5 is a simplified fragmentary elevation, partly in section, ofapparatus adapted for carrying out the method of the present invention,this veiw illustrating the simultaneous vacuum drawing and heatingsteps;

FIG. 6 illustrates the constricting step carried out by the apparatus ofFIG. and

FIG. 7 illustrates the tipping off step carried out by the apparatus ofFIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENT The teachings of the presentinvention are applicable to either percussive or electrically ignitedphotoflash lamps of a wide variety of sizes and shapes. Accordingly,FIGS. 1 and 2 respectively illustrate electrically ignited andpercussive-type photoflash lamps embodying the principles of theinvention.

Referring to FIG. 1, the electrically ignitable lamp comprises anhermetically sealed lamp envelope 2 of glass tubing having a press 4defining one end thereof and an exhaust tip 6 defining the other endthereof. Supported by the press 4 is an ignition means comprising a pairof lead-in wires 8 and 10 extending through and sealed into the press. Afilament l2 spans the inner ends of the lead-in wires, and beads ofprimer l4 and 16 are located on the inner ends of the lead-in wires 8and I0 respectively at their junction with the filament. Typically, thelamp envelope 2 has an internal diameter of less than one-half inch, andan internal volume of less than I cc., although the present invention isequally suitable for application to larger lamp sizes. Acombustion-supporting gas, such as oxygen, and a filamentary combustiblematerial 18, such as shredded zirconium or hafnium foil, are disposedwithin the lamp envelope. Typically, the combustion-supporting gas fillis at a pressure exceeding one atmosphere, with the more recentsubminiature lamp types having oxygen fill pressures of up to severalatmospheres. As will be described in more detail hereinafter, the glassenvelope 2 is reinforced, in accordance with the invention, by avacuumformed thermoplastic coating 20 on its exterior surface.

The percussive-photoflash lamp illustrated in FIG. 2 comprises a lengthof glass tubing defining an hermetically sealed lamp envelope 22constricted at one end to define an exhaust tip 24 and shaped to definea seal 26 about a primer 28 at the other end thereof. The primer 28comprises a metal tube 30, a wire anvil 32, and a charge of fulminatingmaterial 34. A combustible 36, such as filamentary zirconium or hafnium,and a combustion supporting gas, such as oxygen, are disposed within thelamp envelope, with the fill gas being at a pressure of greater than oneatmosphere. As will be detailed hereinafter, the exterior surface ofglass envelope 22 is covered by a vacuum-formed thermoplastic coating 46in accordance with the invention.

The wire anvil 32 is centered within the tube 30 and is held in place bya circumferential indenture 38 of the tube 30 which loops over the head40, or other suitable protuberance, at the lower extremity of the wireanvil. Additional means, such as lobes 42 on wire anvil 32 for example,may also be used in stabilizing the wire anvil, supporting itsubstantially coaxial within the primer tube 30 and insuring clearancebetween the fulminating material 34 and the inside wall of tube 30. Arefractory bead 44 is fused to the wire anvil 32 just above the innermouth of the primer tube 30 to eliminate burn through and function as adeflector to deflect and control the ejection of hot particles offulminating material from the primer. The lamp of FIG. 2 is alsotypically a subminature type having envelope dimensions similar to thosedescribed with respect to FIG. 1.

Although the lamp of FIG. I is electrically ignited, usually from abattery source, and the lamp of FIG. 2 is percussion-ignitable, thelamps are similar in that in each the ignition means is attached to oneend of the lamp envelope and disposed in operative relationship withrespect to the filamentary combustible material. More specifically theigniter filament 12 of the flash lamp in FIG. 1 is incandescedelectrically by current passing through the metal filament support leads8 and I0, whereupon the incandesced filament I2 ignites the beads ofprimer l4 and 16 which in turn ignite the combustible 18 disposed withinthe lamp envelope. Operation of the percussive-type lamp of FIG. 2 isinitiated by an impact onto tube 30 to cause deflagration of thefulminating material 34 up through the tube 30 to ignite the combustible36 disposed within the lamp envelope. The invention is also applicableto other types of electrically ignited lamps, such as those having sparkgap or primer bridge ignition structures.

In accordance with the present invention, we have discovered asolventless, vacuum forming method for applying an optically clearprotective coating on the exterior surface of the glass envelope. Themethod pro vides a significantly faster, safer and more economicalmanufacturing process. and it may be easily integrated on automatedproduction machinery. The process permits use of the stronger, moretemperature resistant thermoplastics, and the resulting coatingmaintains the glass substrate under a compressive load, thereby makingthe glass envelope itself more resistant to thermal shock. As a result,this coating reduces the cost of materials by permitting the use of softglass to meet high containment requirements.

The improved coating method of the invention will now be described withreference to FIGS. 3-7. Referring first to FIG. 3, the thermoplasticmaterial to be coated on the exterior surface of the lamp envelope isinitially provided as a preformed sleeve 48 having the shape of a testtube. To facilitate the one or more metallic members depending from thelamp envelope (i.e. leads 8 and 10, or primer tube 30) one or more holesare provided at the bottom of test tube-shaped sleeve. For purposes ofexample, the method of FIGS. 3-7 will be described with reference tovacuum forming the thermoplastic coating 46 on the percussive lamp ofFIG. 2, although it will be understood that a similar method may beemployed with the electrically ignited lamp of FIG. I. Accordingly,sleeve 48 is provided with a single coaxially disposed hole 50 tofacilitate passage of coaxially projecting primer tube 30. Sleeve 48 maybe formed by a molding or extrusion process, and to minimize possiblechecks and crazes in the plastic upon being vacuum formed to the glassenvelope, the preformed sleeve 48 should be prebaked at about C for atleast I5 minutes to drive away residual moisture prior to assembly withthe glass envelope.

In the next step, shown in FIG. 4, the glass envelope 22 of thepercussive lamp is placed within the preformed thermoplastic sleeve 46,with the primer tube 30 projecting through hole 50. It will be notedthat both the sleeve 48 and the lamp envelope 22 have generally tubularsidewalls. To facilitate the vacuum forming process, the fit should beas close as possible. Accordingly, the outside diameter of the tubularenvelope 22 and the inside diameter of the tubular sleeve 48 aredimensioned so that, when the envelope is placed within the sleeve,there exists a clearance x of from about 0.001 to 0.010 inch between thetubular sidewalls thereof prior to heating and vacuum forming.

The next step, heating and vacuum forming, is illustrated in FIG. 5. Theenvelope and sleeve assembly 22, 48 is held during the evacuating andheating processes by means of a chuck 50 gripping the primer tube 30.Another chuck 52, having an evacuating tube 54, grips the open end ofthe thermoplastic sleeve 48. One or more localized sources of heat,represented by heaters 53, encircle the envelope and sleeve assembly foruniformly applying heat about the tubular sleeve in a substantiallylocalized elevational plane. in operation, the process comprises drawinga vacuum in the space between the sleeve 48, and envelope 22, whilesimultaneously heating the envelope and sleeve assembly incrementallyalong its length. More specifically, the vac uum is drawn through tube54, in the direction of the arrow, at the open end of sleeve 48. At thesame time, the heaters 56 are controlled to heat the sleeve toapproximately the softening temperature of the thermoplastic material. Arelative incremental axial movement is effected between theenvelope-sleeve assembly and the heaters, so that incremental heating ina localized elevational plane starts at the end of the sleeve 48 throughwhich the primer tube 30 projects, and then proceeds toward the open endof the sleeve from which the vacuum is being drawn. In this manner, thetemperature and vacuum cause the thermoplastic sleeve 48 to be formedonto the glass envelope 22 with the interface therebetween substantiallyfree of voids, inclusions and the like.

Referring to FIG. 5, this incremental heating process may beaccomplished at one station by either moving chucks 50 and 52 downwardwith respect to a set of stationary heaters 56, or by moving heaters 56upward with respect to a set of stationary chucks 50 and 52. A preferredmethod of effecting the incremental heating, however, is to index theenvelope-sleeve assembly through a plurality of heating stations, withthe heaters at each station positioned at successively higherelevations.

At the conclusion of the incremental heating process, the sleeve 48 isconstricted at portion 58 (FIG. 6) by slowly pulling chucks 50 and 52away from each other, while continuing to apply heat and draw a vacuum.Finally, as shown in FIG. 7, the vacuum formed sleeve 48 on the lamp isseparated from the portion 60 of the sleeve held in chuck 52 and tippedoff at point 62, thereby completing the encapsulation of glass envelope22 in the thermoplastic coating 46.

The composition of sleeve 48, and thus coating 46, may be of any vacuumformable light-transmitting thermoplastic material having a reasonablyhigh impact strength and softening temperature. Suitable materialsinclude acrylic, acrylonitrile-butadiene-styrene, cellulose acetate,ionomers, methylpentene polymer, nylon, polycarbonate, polystyrene,polysulfone, or alloys thereof. In the case of some of the hardermaterials, it may also be desirable to add a small amount (10-20%) of acompatible plasticizer to the composition. Further, commerical blue dyescan be used in the sleeve, or coating, for color corrections desirablewith various photographic color film.

Preferably, the thermoplastic material is selected to have a coefficientof thermal expansion several times greater than the coefficient ofthermal expansion of the glass envelope. in this manner, the coating 46,provided by the above described vacuum forming process, will exert acompressive load on the glass envelope 22 to thereby in effectstrengthen the glass and make it more resistant to thermal shock. Forexample, with a coefficient of thermal expansion at least six timesgreater than that for the glass, the thermoplastic coating may exert acompressive load of from about 000 to about 4000 pounds per square inchon the glass envelope.

The added containment strength provided by this compressive loading maybe better understood by briefly considering the effects of thecombustion process. Upon flashing the lamp and igniting the shreds ofcombustible, the inner surface of the glass envelope is subjected tosevere thermal shock in the form of impact from hot globules of metaloxide; for example, zirconium oxide has a melting point of 2715C. Eachthermal impact against the internal glass surface produces a thermalstress gradient through the wall of the glass envelope, which serves asan insulator to the conducted heat, and causes expansion of the glass.any thermoplastic coating on the glass will be under tension (T,,) andthere will be localized tensile stress (T at the interface of thecoating and glass, opposite the point of globule impact. The build up ofthe localized tensile stress T, by the thermal stress gradient is whatcan eventually cause a crack through the glass wall. On the other hand,the compression loading (C) which is exerted on the glass envelope bythe coating functions to counteract the tensile loading of T and T, bydelaying the thermal stress gradient through the glass wall; this may beillustrated as T T, C. Accordingly, the higher the compressive loading,the stronger the glass. Also, however, an increase in the compressiveloading on the glass results in a corresponding increase in the tensileloading on the coating. Hence. a compressive load that is too high canbe detrimental. Where necessary, the compressive loading can be relievedby the inclusion of a small amount of plasticizer in the coatingcomposition and/or fillers that alter the thermal expan sioncoefficient.

Another aspect of this encapsulated lamp structure to be noted is thatthe vacuum forming process leaves a well defined interface between theenvelope and the coating-the coating is not sealed to the envelope. As aresult, a crack in the glass envelope will stop at the interface and becontained by the thermoplastic coatmg.

In one typical embodiment of the invention, a percussive flashlamp ofthe type shown in FIG. 2 was provided with a clear vacuum-formed coating46 of polycarbonate resin having a wall thickness of about 0.020 inch.The lamp contained a combustible fill 36 comprising 18.5 mgs. ofshredded zirconium foil and oxygen at a fill pressure of 20 atmospheres.The tubular envelope 22 was formed of 6-] type soft glass and had anominal outside diameter of 0.250 inch. in the process of coating thelamp, an injection molded sleeve 48 of clear polycarbonate resin havinga nominal inside diameter of 0.260 inch and a wall thickness of 0.020was employed. During vacuum forming, the molded sleeve was incrementallyheated to a temperature of about 400F by a nitrogen flow serpentineheater. The coefficient of thermal expansion of soft glass of this typeranges from 35 to X l0 in./in./C between 20 and 300C, whereas thecoefficient of thermal expansion of unfilled polycarbonate between 25and 140C is about 660 X lin./in./C. Upon measuring several sections oflamps made as described above, the average compressive stress exerted bythe coating 46 upon the glass envelope 22 was found to be about 1392pounds per square inch. Flashing of a number of these lamps in both thevertical and horizontal position exhibited no containt ..ent failures.

Although the invention has been described with re spect to specificembodiments, it will be appreciated that modifications and changes maybe made by those skilled in the art without departing from the truespirit and scope of the invention.

What we claim is:

l. A method of coating the glass envelope ofa photoflash lamp with alight-transmitting thermoplastic material, said method comprising:

placing said glass envelope within a preformed sleeve of saidthermoplastic material, and

drawing a vacuum in the space between said thermoplastic sleeve and saidglass envelope, while simultaneously heating said sleeve and envelopeassembly incrementally along the length-thereof so that the temperatureand vacuum cause said thermoplastic sleeve to 1 be incrementally formedwith a substantially uniform thickness onto the entire exterior surfaceof said glass envelope with the interface therebetween substantiallyfree of voids, inclusions and the like, whereby after cooling, saidthermoplastic coating exerts a compressive load on said glass envelopeand a stronger envelope structure is obtained for providing improvedcontainment during flashing.

2. The method of claim 1 wherein heat is applied uniformly about saidsleeve and envelope assembly by one or more localized sources of heatencircling said assembly, and a relative incremental axial movement iseffected between said assembly and said one or more sources of heat.

3. The method of claim 1 wherein heat is applied uniformly about saidsleeve and envelope assembly by one or more localized sources of heatencircling said assembly, and said assembly is indexed through aplurality of heating stations, each of which has said one or morelocalized sources of heat positioned at successively higher elevations.

4. The method of claim 1 wherein the heating of said sleeve and envelopeassembly is controlled to bring said thermoplastic sleeve toapproximately the softening temperature thereof.

5. The method of claim 1 including the further step of prebaking saidthermoplastic sleeve to remove the moisture therefrom prior toassembling said sleeve with said glass envelope.

6. The method of claim 5 wherein said prebaking of the sleeve isconducted at a temperature of about 125C for at least minutes.

7. The method of claim 1 including the further steps of constricting andtipping off said vacuum-formed sleeve at the conclusion of saidincremental heating process.

8. The method of claim 1 wherein both said thermoplastic sleeve and saidglass envelope have generally tubular sidewalls, and the outsidediameter of said tubu lar envelope and inside diameter of said tubularsleeve are dimensioned so that, when said envelope is placed within saidsleeve, there exists a clearance of from about 0.001 to 0.010 inchbetween the tubular sidewalls thereof prior to heating.

9. The method of claim 1 wherein said lamp has at least one metallicmember depending from said glass envelope, said preformed thermoplasticsleeve is shaped like a test tube having at least one hole in the bottomthereof, and in said step of placing said glass envelope within saidsleeve, said metallic member is inserted through said hole to therebyproject outside said test tube-shaped sleeve.

10. The method of claim 9 wherein said vacuum is drawn at the open endof said test tube-shaped sleeve opposite the end through which saidmetallic member projects, and said incremental heating of said assemblyproceeds from the end of said sleeve through which said metallic memberprojects toward the open end of said sleeve from which said vacuum isbeing drawn.

11. The method of claim 10 wherein said envelope and sleeve assembly isheld during said evacuating and heating processes by means of a chuckgripping said projecting metallic member.

12. The method of claim 11 wherein said photoflash lamp is apercussive-type, and said metallic member comprises a primer tubecoaxially projecting from one end of said envelope.

13. The method of claim 1 wherein the coefficient of thermal expansionof said thermoplastic sleeve is at least about six times greater thanthe coefficient of thermal expansion of said glass envelope.

14. The method of claim 1 wherein the composition of said sleevecomprises a light-transmitting thermoplastic selected from the groupconsisting of acrylic, acrylonitrilebutadiene-styrene, celluloseacetate, ionomers, methylpentene polymer, nylon, polycarbonate,polystyrene, polysulfone, and alloys thereof.

15. The method of claim 14 wherein the composition of said sleevefurther includes a small amount of plasticizer.

16. The method of claim 1 wherein said thermoplastic sleeve comprises apolycarbonate resin.

17. The method of claim 16 wherein the thickness of said polycarbonatesleeve is about 0.020 inch.

18. The method of claim 10 wherein: both said sleeve and said envelopehave generally tubular sidewalls; the outside diameter of said tubularenvelope and the inside diameter of said tubular sleeve are dimensionedso that, when said envelope is placed within said sleeve, there exists aclearance of from about 0.001 to 0.010 inch between the tubularsidewalls thereof prior to heating; and the heating of said sleeve andenvelope assembly is controlled to bring said thermoplastic sleeve toapproximately the softening temperature thereof; and including thefurther steps of prebaking said thermoplastic sleeve at a temperature ofabout C for at least 15 minutes to remove the moisture therefrom priorto assembling said sleeve with said glass envelope, and constricting andtipping off said vacuum formed sleeve at the conclusion of saidincremental heating process.

19. A method of coating the glass envelope of a photoflash lamp with alight-transmitting thermoplastic material, said method comprising:

placing said glass envelope within a preformed sleeve of saidthermoplastic material, and

forming said sleeve with a substantially uniform thickness onto theentire exterior surface of said glass envelope by heating said sleeveand drawing a vacuum in the space between said sleeve and said glassenvelope. whereby after cooling, said thermoplastic coating exerts acompressive load on said glass envelope and a stronger envelopestructure is obtained for providing improved containment duringflashing.

20. The method of claim 19 including the further step of prebaking saidthermoplastic sleeve to remove the moisture therefrom prior toassembling said sleeve with said glass envelope.

21. The method of claim 20 wherein said prebaking of the sleeve isconducted at a temperature of about 125C for at least minutes.

22. The method of claim 19 including the further steps of constrictingand tipping off said formed sleeve at the conclusion of said formingprocess.

23. The method of claim 19 wherein both said thermoplastic sleeve andglass envelope have generally tubular sidewalls.

24. The method of claim 19 wherein said lamp has at least one metallicmember depending from said glass envelope, said preformed thermoplasticsleeve is shaped like a test tube having at least one hole in the bottomthereof, and in said step of placing said glass envelope within saidsleeve, said metallic member is inserted through said hole to therebyproject outside said test tube shaped sleeve.

25. The method of claim 24 wherein the heating of said test-tube shapedsleeve starts at the end thereof through which said metallic memberprojects and pro ceeds toward the open end of the sleeve, and saidvacuum is drawn at said open end of the sleeve.

l l 1 t l

1. A method of coating the glass envelope of a photoflash lamp with alight-transmitting thermoplastic material, said method comprising:placing said glass envelope within a preformed sleeve of saidthermoplastic material, and drawing a vacuum in the space between saidthermoplastic sleeve and said glass envelope, while simultaneouslyheating said sleeve and envelope assembly incrementally along thelength-thereof so that the temperature and vacuum cause saidthermoplastic sleeve to be incrementally formed with a substantiallyuniform thickness onto the entire exterior surface of said glassenvelope with the interface therebetween substantially free of voids,inclusions and the like, whereby after cooling, said thermoplasticcoating exerts a compressive load on said glass envelope and a strongerenvelope structure is obtained for providing improved containment duringflashing.
 2. The method of claim 1 wherein heat is applied uniformlyabout sAid sleeve and envelope assembly by one or more localized sourcesof heat encircling said assembly, and a relative incremental axialmovement is effected between said assembly and said one or more sourcesof heat.
 3. The method of claim 1 wherein heat is applied uniformlyabout said sleeve and envelope assembly by one or more localized sourcesof heat encircling said assembly, and said assembly is indexed through aplurality of heating stations, each of which has said one or morelocalized sources of heat positioned at successively higher elevations.4. The method of claim 1 wherein the heating of said sleeve and envelopeassembly is controlled to bring said thermoplastic sleeve toapproximately the softening temperature thereof.
 5. The method of claim1 including the further step of prebaking said thermoplastic sleeve toremove the moisture therefrom prior to assembling said sleeve with saidglass envelope.
 6. The method of claim 5 wherein said prebaking of thesleeve is conducted at a temperature of about 125*C for at least 15minutes.
 7. The method of claim 1 including the further steps ofconstricting and tipping off said vacuum-formed sleeve at the conclusionof said incremental heating process.
 8. The method of claim 1 whereinboth said thermoplastic sleeve and said glass envelope have generallytubular sidewalls, and the outside diameter of said tubular envelope andinside diameter of said tubular sleeve are dimensioned so that, whensaid envelope is placed within said sleeve, there exists a clearance offrom about 0.001 to 0.010 inch between the tubular sidewalls thereofprior to heating.
 9. The method of claim 1 wherein said lamp has atleast one metallic member depending from said glass envelope, saidpreformed thermoplastic sleeve is shaped like a test tube having atleast one hole in the bottom thereof, and in said step of placing saidglass envelope within said sleeve, said metallic member is insertedthrough said hole to thereby project outside said test tube-shapedsleeve.
 10. The method of claim 9 wherein said vacuum is drawn at theopen end of said test tube-shaped sleeve opposite the end through whichsaid metallic member projects, and said incremental heating of saidassembly proceeds from the end of said sleeve through which saidmetallic member projects toward the open end of said sleeve from whichsaid vacuum is being drawn.
 11. The method of claim 10 wherein saidenvelope and sleeve assembly is held during said evacuating and heatingprocesses by means of a chuck gripping said projecting metallic member.12. The method of claim 11 wherein said photoflash lamp is apercussive-type, and said metallic member comprises a primer tubecoaxially projecting from one end of said envelope.
 13. The method ofclaim 1 wherein the coefficient of thermal expansion of saidthermoplastic sleeve is at least about six times greater than thecoefficient of thermal expansion of said glass envelope.
 14. The methodof claim 1 wherein the composition of said sleeve comprises alight-transmitting thermoplastic selected from the group consisting ofacrylic, acrylonitrilebutadiene-styrene, cellulose acetate, ionomers,methylpentene polymer, nylon, polycarbonate, polystyrene, polysulfone,and alloys thereof.
 15. The method of claim 14 wherein the compositionof said sleeve further includes a small amount of plasticizer.
 16. Themethod of claim 1 wherein said thermoplastic sleeve comprises apolycarbonate resin.
 17. The method of claim 16 wherein the thickness ofsaid polycarbonate sleeve is about 0.020 inch.
 18. The method of claim10 wherein: both said sleeve and said envelope have generally tubularsidewalls; the outside diameter of said tubular envelope and the insidediameter of said tubular sleeve are dimensioned so that, when saidenvelope is placed within said sleeve, there exists a clearance of fromabout 0.001 to 0.010 inch between the tubular sidewalls thereof prior toheating; and the heating of said sleeve and envelope assembly iscontrolled to bring said thermoplastic sleeve to approximately thesoftening temperature thereof; and including the further steps ofprebaking said thermoplastic sleeve at a temperature of about 125*C forat least 15 minutes to remove the moisture therefrom prior to assemblingsaid sleeve with said glass envelope, and constricting and tipping offsaid vacuum formed sleeve at the conclusion of said incremental heatingprocess.
 19. A METHOD OF COATING THE GLASS ENVELOPE OF A PHOTOFLASH LAMPWITH A LIGHT-TRANSMITTING THERMOPLASTIC MATERIAL, SAID METHODCOMPRISING: PLACING SAID GLASS ENVELOPE WITHIN A PREFORMED SLEEVE OFSAID THERMOPLASTIC MATERIAL, AND FORMING SAID SLEEVE WITH ASUBSTANTIALLY UNIFORM THICKNESS ONTO THE ENTIRE EXERIOR SURFACE OF SAIDGLASS ENVELOPE BY
 20. The method of claim 19 including the further stepof prebaking said thermoplastic sleeve to remove the moisture therefromprior to assembling said sleeve with said glass envelope.
 21. The methodof claim 20 wherein said prebaking of the sleeve is conducted at atemperature of about 125*C for at least 15 minutes.
 22. The method ofclaim 19 including the further steps of constricting and tipping offsaid formed sleeve at the conclusion of said forming process.
 23. Themethod of claim 19 wherein both said thermoplastic sleeve and glassenvelope have generally tubular sidewalls.
 24. The method of claim 19wherein said lamp has at least one metallic member depending from saidglass envelope, said preformed thermoplastic sleeve is shaped like atest tube having at least one hole in the bottom thereof, and in saidstep of placing said glass envelope within said sleeve, said metallicmember is inserted through said hole to thereby project outside saidtest tube shaped sleeve.
 25. The method of claim 24 wherein the heatingof said test-tube shaped sleeve starts at the end thereof through whichsaid metallic member projects and proceeds toward the open end of thesleeve, and said vacuum is drawn at said open end of the sleeve.